In known ignition systems of the above-mentioned type, storage (ROM) is provided in which all the values for approximating a family of curves are stored. Specifically, the family of curves consists of a plurality of curves of ignition timing advance v. engine speed for different values of a predetermined engine parameter such as the pressure in the intake manifold. The capacity of the storage must be such that all values for each curve can be stored. The requirements for the number of storage locations in the ROM thus is a function of the accuracy which is required. At a sufficient accuracy, the ignition timing advance can be very exactly fitted to the particular then-present operating condition of the engine.